--- title: "Zones and backup for breaker 7 failure" subject: "Switchgear and Protection" chapter: 1 key: "semester_6/switchgear_and_protection:zones-backup-breaker-7" source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/zones-backup-breaker-7 site: BCT Engineering --- # Zones and backup for breaker 7 failure Switchgear and Protection · Chapter 1: Principle of Power System Protection - Asked in: 2074 Bhadra, 6 marks - Video: [Lec (5) Part (7): Principles of Breaker Failure](https://www.youtube.com/watch?v=pPV2p6XNS74) — Prof. Eng. Courses - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/zones-backup-breaker-7 - Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-1 ## Question Consider the system portion shown in figure below. (i) Sketch the zone of protection. (ii) Describe a possible backup scheme for failure of breaker 7 for a fault on line BC; is load 1 interrupted? (iii) Describe a means of clearing a fault on line BC without a momentary interruption of load 1. [Figure: Bus A is fed through breaker 1. Two parallel lines run from bus A to bus B: the upper line has breaker 2 at A and breaker 4 at B, the lower line has breaker 3 at A and breaker 5 at B. Load 1 is supplied from bus B through breaker 6. Line BC leaves bus B through breaker 7 and reaches bus C, from which Load 2 is supplied through breaker 8.] ## Short answer Zones: bus A (1,2,3), line AB1 (2,4), line AB2 (3,5), bus B (4,5,6,7), line BC (7,8), load 1 (6), load 2 (8). If breaker 7 fails, breaker-failure relay trips 4,5,6; load 1 is interrupted. To avoid interruption, use double-bus with bus coupler or two series breakers on BC, so bus B remains healthy and load 1 stays fed. ## Answer Assumption: breakers 1–8 are as described; each line has a breaker at both ends and bus B connects breakers 4, 5, 6 and 7. #### (i) Zones of protection ```text [1] ┌─────┼──────── Bus A zone ──────┐ │ ───┴──┬──────────────┬─── │ └────────┼──────────────┼────────┘ [2] [3] ┌ Line AB1 ┐ ┌ Line AB2 ┐ │ │ │ │ │ │ └───[4]────┘ └───[5]────┘ ┌────────┼──────────────┼────────┐ │ ──────┴───┬──────┬───┴── │ Bus B zone └───────────[6]────[7]───────────┘ Load 1 ┌──┼────────┐ Line BC │ │ │ zone └─[8]───────┘ │ Bus C / Load 2 Each zone overlaps its neighbours at every breaker (1,2,3,4,5,6,7,8). ``` Zones: bus A (1, 2, 3); line AB-1 (2, 4); line AB-2 (3, 5); bus B (4, 5, 6, 7); line BC (7, 8); load 1 feeder (6); load 2 / bus C (8). #### (ii) Back-up for failure of breaker 7 (fault on line BC) - Normally relays of line BC trip breakers 7 and 8. - If breaker 7 fails to open, the fault is still fed from bus B. - **Local back-up (breaker failure protection):** after about 150–200 ms, the breaker-failure relay of 7 trips all other breakers connected to bus B: **4, 5 and 6**. - **Remote back-up (alternative):** zone-2/zone-3 distance relays or time-graded overcurrent relays at bus A trip **2 and 3**. - In both cases bus B is de-energised, so **load 1 is interrupted** (in the local scheme breaker 6 is tripped; in the remote scheme bus B loses supply). #### (iii) Clearing BC fault without momentary interruption of load 1 The fault current from bus B must be interrupted by a breaker other than the one that isolates load 1. Possible means: - Provide **two breakers in series** on the BC line at B (or a breaker-and-a-half / ring bus arrangement), so that if breaker 7 fails the second breaker clears the fault and bus B remains healthy. - Use a **double-bus with bus coupler** arrangement: line BC and load 1 on different buses; the breaker-failure relay of 7 trips only the bus coupler and the breakers of BC's bus. With these arrangements a fault on BC, even with a stuck breaker, is removed without de-energising the bus that supplies load 1. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. If breaker 7 fails for a BC fault, which breakers does breaker-failure protection trip? A. 4, 5 and 6 B. 1, 2 and 3 C. 8 and 9 only D. Only 7 again Answer: A. It trips all other breakers on bus B after about 150-200 ms. (Key point: Local back-up) 2. Is load 1 interrupted in this back-up scheme? A. No, it stays fed through breaker 6 B. No, because breaker 8 supplies it C. Yes, because bus B is de-energised D. Only if line AB fails Answer: C. Tripping bus B breakers (including 6) interrupts load 1. (Key point: Load 1) 3. What is the remote back-up alternative? A. Relays at bus C trip breaker 9 B. Distance zone-2/3 or time-graded overcurrent relays at bus A trip 2 and 3 C. Breaker 6 trips alone D. Nothing trips Answer: B. Upstream relays at A act as delayed back-up. (Key point: Remote back-up) 4. How can a BC fault be cleared without interrupting load 1? A. Using a single bus with one breaker B. Increasing relay delay C. Removing breaker 8 D. Two breakers in series on BC at B (or breaker-and-a-half/ring), or double bus with coupler Answer: D. A second breaker clears the fault so bus B stays healthy. (Key point: No interruption) ## Related in Switchgear and Protection - [Ring bus rearrangement and zones](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aring-bus-comparison) — Switchgear and Protection, chapter 1 - [Protection requirements; protective system vs scheme](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aprotective-system-vs-scheme) — Switchgear and Protection, chapter 1 - [Requirements, coordination and protection zone](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Arequirements-coordination-zone) — Switchgear and Protection, chapter 1 - [Methods of providing backup protection](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Amethods-backup-protection) — Switchgear and Protection, chapter 1 - [Fault clearing process with trip circuit](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Afault-clearing-trip-circuit) — Switchgear and Protection, chapter 1 - [Zones of protection and their rules](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Azones-of-protection) — Switchgear and Protection, chapter 1, asked 2× ## Related in other subjects - [Critical clearing time and angle, fault at P](https://www.bctengineer.com/question.md?key=semester_5%2Fpower_system_analysis_ii%3Acritical-clearing-time-point-p-h5) — Power System Analysis II, chapter 6 - [Critical clearing angle/time, fault at infinite bus](https://www.bctengineer.com/question.md?key=semester_5%2Fpower_system_analysis_ii%3Acritical-clearing-infinite-bus-h5) — Power System Analysis II, chapter 6 - [Fault at load end of overhead line](https://www.bctengineer.com/question.md?key=semester_5%2Fpower_system_analysis_ii%3Afault-load-end-33kv-line) — Power System Analysis II, chapter 3 - [Critical clearing, fault at infinite bus, H = 6](https://www.bctengineer.com/question.md?key=semester_5%2Fpower_system_analysis_ii%3Acritical-clearing-fault-infinite-bus-h6) — Power System Analysis II, chapter 6